The present invention relates to an implantable cardiac stimulation system using an automatic capture feature. More specifically, the present invention relates to an implantable cardiac stimulation system in which an external programmer controls an automatic capture calibration routine and displays pertinent information regarding the feasibility of performing automatic capture.
Implantable cardiac stimulating devices including pacemakers, cardioverters and defibrillators, detect and treat incidents of cardiac arrhythmias. Such devices are coupled to a patient""s heart through transvenous leads that are used to sense electrical signals from the heart and deliver both low voltage and high voltage electrical therapy to the heart. The device circuitry generally includes sensing circuitry for sensing cardiac electrical activities in order to detect intrinsic electrical depolarizations of the cardiac tissue that cause contraction of the respective heart chambers.
In the atria, detection of a P-wave indicates atrial contraction, and in the ventricles detection of an R-wave, also known as a QRS complex, indicates ventricular contraction. If detection of an intrinsic P-wave or an R-wave does not occur within a given interval of time, generally referred to as the xe2x80x9cescape interval,xe2x80x9d the heart rate is determined as being too slow. A stimulation pulse is then generated by the pacemaker circuitry and delivered to the appropriate heart chamber at the end of the escape interval in order to stimulate the muscle tissue of the heart to contract, thus maintaining a minimum heart rate. The duration of the escape interval corresponds to some base pacing rate, for example an escape interval of 1,200 msec would maintain a base pacing rate of 50 heart beats per minute.
The electrical depolarization caused by the delivery of a stimulation pulse is known as an xe2x80x9cevoked response.xe2x80x9d An evoked response will only occur when the stimulating pulse is of sufficient energy to cause depolarization of the cardiac tissue, a condition known as xe2x80x9ccapture.xe2x80x9d The minimum stimulating energy required to capture a chamber of the heart is known as xe2x80x9cthreshold.xe2x80x9d
Modern pacemakers often include a feature known as xe2x80x9cautomatic capture.xe2x80x9d When the automatic capture feature is implemented, the pacemaker circuitry detects the evoked response following the delivery of a stimulation pulse in order to verify that capture has occurred. If no evoked response is detected, the stimulation pulse may have been of insufficient energy to capture the heart; therefore, a high-energy back-up pulse is quickly delivered to the heart in order to maintain the desired heart rate. A threshold detection algorithm is then invoked in order to re-determine what minimum energy is required to capture the heart.
The stimulating pulse energy is automatically adjusted to this new threshold value plus some safety margin. As long as an evoked response is detected following a stimulation pulse, that is, as long as capture is verified, pacing will continue at the set pulse energy.
Hence, the automatic capture feature improves pacemaker performance in at least two ways: 1) it verifies that the stimulation therapy delivered has been effective in causing the heart chamber activation, and 2) it improves battery energy longevity by determining the lowest stimulation energy needed to effectively capture the heart.
However, one problem with capture detection is that the signal sensed by the ventricular and/or atrial sensing circuits immediately following the application of a stimulation pulse may not be an evoked response. Rather, it may be noise, either electrical noise caused, for example, by electromagnetic interference (EMI), or myocardial noise caused by random myocardial or other muscle contractions (muscle xe2x80x9ctwitchingxe2x80x9d). Alternatively, the signal sensed by the ventricular and/or atrial sensing circuits may be a natural R-wave or P-wave that just happens to occur immediately following the application of a non-capturing stimulation pulse.
Another problematic condition is xe2x80x9cfusionxe2x80x9d. Fusion occurs when a pacing pulse is delivered such that the evoked response occurs coincidentally with an intrinsic depolarization. The evoked signal may be absent or altered preventing correct capture detection by the pacemaker""s capture detection algorithm. A loss of capture may be indicated when capture is in fact present, which is an undesirable situation that will cause the pacemaker to unnecessarily deliver a high-energy back-up pacing pulse and to invoke the threshold testing function in a chamber of the heart. Frequent delivery of back-up pacing pulses or execution of threshold tests defeats the purpose of the energy-saving features of autocapture. If fusion continues during a threshold test, the pacing energy output may be driven to a maximum level, quickly depleting the battery energy.
The incidence of fusion can be particularly problematic in patients with intermittent or intact atrio-ventricular conduction being treated by dual chamber pacing. In dual chamber pacing, both atrial and ventricular activity are monitored. A P-wave detected in the atria is followed by an AV/PV interval which is the desired delay between an atrial depolarization and a ventricular depolarization. If an intrinsic R-wave is not detected prior to expiration of the AV/PV delay, a Vpulse is delivered to pace the ventricles. Since the AV conduction time may vary, an intrinsically conducted R-wave may occur at different times and therefore may occur approximately the same time that a ventricular pacing pulse is delivered. Furthermore, the AV/PV interval may be programmed inappropriately leading to increased likelihood of fusion events. Fusion masquerading as loss of capture will cause the pacemaker to initiate frequent threshold tests and may drive the pacemaker to its maximum pacing output.
Yet another signal that interferes with the detection of an evoked response is associated with lead electrode polarization. Lead electrode polarization is caused by electrochemical reactions that occur at the lead/tissue interface due to the application of the electrical stimulation pulse across such interface. The lead polarization signal is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy, and many other variables.
The evoked response is monitored within 3 to 80 msec of the stimulation pulse. During the early portion of this time, the lead polarization signal voltage is still relatively high. In order to minimize lead polarization voltage, low polarization materials can be used in manufacturing the electrode. Still, since the evoked response and polarization signal occur simultaneously, if the polarization signal is very high, it may not be possible to reliably detect an evoked response. The result may be a false positive detection of the evoked response. Such false positive detection leads to a false capture indication, which, in turn, can lead to missed heartbeats, a highly undesirable situation.
Variation in the lead polarization signal can be significant from patient to patient depending on implanted lead configurations and other factors. Therefore, calibration methods are generally required to determine a threshold for detecting the evoked response and distinguishing it from the lead polarization signal.
Different parameters or characteristics of the evoked response have been proposed in automatic capture calibration and automatic capture detection schemes, including impedance change, voltage differential (dV/dt), signal polarity reversal, and peak negative amplitude.
Typically, evoked response sensing occurs between the tip and ring of a bipolar lead connected to the device sensing circuits. The evoked response may also be monitored by sensing between the ring electrode and device housing. In either event, a bipolar pacing lead has generally been required in order to detect the evoked response. These configurations have been selected since they reduce the likelihood of false positive capture detection. Such reduction is achieved by selecting a feature in the bipolar evoked response that is not strongly expressed in the polarization artifact. Alternatively, in the ring-case configuration, one of the pacing electrodes may be removed from the sensing circuit, thereby reducing the sensed polarization signal. This implies that these automatic capture schemes will not work with unipolar pacing leads.
Thus, in patients having unipolar leads implanted in conjunction with a cardiac stimulation device, the ability to reliably employ the automatic capture feature has been heretofore limited.
It would thus be desirable to provide the cardiac stimulation system with an automatic calibration routine that evaluates variables associated with the lead polarization signal and the evoked response signal, and, based on this evaluation, determines whether or not the automatic capture feature is recommended for a particular patient. In addition, it would be desirable to report these calibration variables to the physician to allow him or her to make informed decisions in enabling or disabling the automatic capture feature and, when enabled, to make an informed decision in selecting appropriate automatic capture operating parameters.
The present invention addresses this and other needs by providing an implantable cardiac stimulation system with an automatic capture calibration feature capable of automatically evaluating whether the automatic capture can be reliably performed using a unipolar electrode configuration, and further calculating and reporting calibration variables that can be used by a medical practitioner in programming automatic capture parameters. It should be understood that the calibration procedure of the present invention can alternatively be performed using leads with bipolar or multipolar electrode configurations.
The implementation of this calibration procedure in an external programmer in communication with an implanted stimulation device allows thorough testing and evaluation of stimulation response signals, and gives the opportunity to display pertinent information regarding the feasibility of the automatic capture feature to the physician.
The automatic calibration procedure of the present invention, which employs the xe2x80x9cpaced depolarization integral method calibrationxe2x80x9d or xe2x80x9cPDI-method calibrationxe2x80x9d, can be executed by an external programmer controlling certain operations of an implanted cardiac stimulation device. The calibration procedure includes the following steps: 1) automatic gain adjustment and fusion avoidance adjustment; 2) paced depolarization integral data collection and table creation; 3) capture threshold determination and stimulation response curve slope determination; 4) automatic capture recommendation and failure conditions reporting; and 5) reporting of calibration variable estimations.
When the automatic calibration procedure is initiated, the external programmer first adjusts the gain of the sensing circuit (or circuits) of the implanted stimulation device to achieve a desired maximum magnitude of sampled signals. If necessary, the programmer also recommends adjustment of pacer timing parameters in order to minimize the likelihood of fusion, such as increasing the base rate of the pacemaker, or shortening the AV and or PV intervals.
The programmer then collects data relating to the lead polarization signal and the evoked response signal following the delivery of a stimulation pulse by integrating a sampled cardiac EGM signal. In so doing, the programmer triggers the implanted stimulation device to deliver a given number of stimulation pulses at a several pulse amplitude settings over a specified range, including both supra-capture threshold amplitudes and sub-capture threshold amplitudes. The sensed cardiac signal is integrated in order to obtain the paced depolarization integral (or PDI) associated with each stimulation pulse. The values of the integrals obtained for a given number of stimulating pulses at each pulse amplitude are statistically evaluated, and the results are stored in memory. If inappropriate or insufficient results are obtained during this data collection and analysis, the physician is alerted that the calibration procedure cannot be run.
Based on the paced depolarization integral data collected, the capture threshold is determined, and the slope of the stimulation response curve (pulse amplitude versus paced depolarization integral) is calculated. Based on further mathematical analysis of the paced depolarization integral data, which in essence determines the margin for safely discriminating an evoked response signal from a lead polarization signal, the activation of the automatic capture feature would be determined to be either xe2x80x98recommendedxe2x80x99 or xe2x80x98not recommended.xe2x80x99
For either condition, a set of calibration variables are calculated and displayed, including minimum and maximum evoked response amplitudes, maximum polarization signal amplitude, evoked response sensitivity, and evoked response safety margin. Having this information, the medical practitioner can make informed decisions in programming the automatic capture feature of an implantable cardiac stimulation device.
Thus, one feature of the present invention is a calibration procedure that is executed by an external programmer controlling certain operations of the implanted stimulation device. Another feature of the present invention included in this calibration procedure is the ability to make gain adjustments or fusion avoidance adjustments to improve the ability of the calibration procedure in collecting sufficient and appropriate data.
A further feature of the present invention is the ability to sample and digitize a cardiac stimulation response signal. Yet another feature of the present invention is a method for integrating the sampled stimulation response signal during a defined response time window and relative to an integration baseline in order to obtain a paced depolarization integral.
Still another feature of the present invention is the performance of mathematical and statistical analyses of the paced depolarization integral data in order to determine the capture threshold and by what margin automatic capture can be reliably performed. A further feature is the calculation of estimations of certain variables pertinent to the performance of automatic capture. Another feature is a method for displaying calibration procedure failure conditions, an automatic capture recommendation, and automatic capture variable estimations.